Slow Transformation Products: of Radium. 637 
hours, the residual activity is less than one millionth of the 
activity immediately after removal. Griesel* also observed 
that a platinum wire, after exposure to the emanation, showed 
residual activity which, he states, consists only of @ rays. 
An account will now be given of some investigations made 
by the writer on the nature of this residual activity and the 
chemical properties of the active matter itself. It is first of 
all necessary to show that the residual activity arises in con- 
sequence of a deposit of radioactive matter, and is not due 
to some action of the intense radiations to which the body 
made active has been subjected. 
The inside of a long glass tube was covered with equal 
areas of thin metal, including aluminium, iron, copper, silver, 
lead, and platinum. A large amount of radium emanation 
was introduced into the tube, and the tube closed. After 
seven days the metal plates were removed, and, after allowing 
two days to elapse for the ordinary excited activity to 
disappear, the residual activity of the plates was tested by an 
electrometer. The activity of the plates was found to be 
unequal, being greatest for copper and silver, and least for 
aluminium. ‘The activity of copper was twice as great as 
| that of aluminium. After standing for another week, the 
activity of the plates was again tested. The activity of each 
: had diminished in the interval to some extent, but the initial 
differences observed had to a large extent disappeared. 
After reaching a minimum value the activity of each plate 
| slowly but steadily increased at the same rate. After a 
: month’s interval the activity of each of the plates was nearly 
the same, and over three times the minimum value. The 
initial irregularities in the decay curves of the different 
| metals are, in all probability, due to slight but different degrees 
| of absorption of the radium emanation by the metal plates, the 
: absorption being greatest for copper and silver and least for 
aluminium. As the occluded emanation was slowly released 
or lost its activity, the activity of the metal fell to a limiting 
value. The absorption of the radium emanation by lead, 
paraffin, and caoutchouc was some time ago observed by 
Curie and Danne ft. 
The residual activity on the plates comprised both « and 6 
rays, the latter being present, in all cases, in a very unusual 
proportion. The equality of the activity and the identity of 
the radiation emitted from each plate shows that the residual 
activity is due to changes of some form of matter deposited 
on the plates, and that it cannot be ascribed to an action of 
* Berichte d. D. Chem. Gesell. p. 2368 (1903). 
+ Comptes Rendus, cxxxvi. p. 364 (1903). 
Phil. Mag. 8S. 6. Vol. 8. No. 47. Nov. 1904, 2X 
